Removed unnecessary forward declarations

The full definitions of these structs are already included with Box2D.h,
and is necessary because the classes are used as direct members.

This also removes a compiler warning with Visual Studio and Clang about
the class/struct mismatch.
Removed mGravityScale because it's already part of b2BodyDef
Removed unnecessary explicit calls to member constructors

They will be called implicitly exactly the same way.
Fixed many code style issues

* Member functions start with lower-case character
* Space in 'if {' and 'foreach {'
* No newline between 'if' and '{'
* Marked many getters as 'const'
* Removed some 'get' from getters, not Qt style. Exceptions I've kept
  are generally dynamically changing members that can't (easily) be
  turned into properties.
* Placement of space in 'Class *class'
* Space after commas in parameter list
Merge pull request #9 from bjorn/master

Some general cleanups